A computational comparison of key mechanistic water oxidation steps using mononuclear manganese catalysts

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The evolutionary advance of a Mn4Ca oxygen-evolving complex within Photosystem II (PSII) allows for light-dependent splitting of water to produce protons and oxygen throughout photosynthesis. The oxidation mechanism related to the manganese containing enzymatic core of the complex may provide insight into how synthetic mononuclear manganese water oxidation catalysts could be designed and investigated. Specifically, the mechanistic step involving the oxygen-oxygen bond formation from water is of particular interest. This step is thought to be critical for dioxygen evolution in PSII as well as synthetic mimics of water photooxidation. Using density functional theory (DFT using B3LYP exchange-correlation function) we compare the energetics and local bonding interactions of working mononuclear manganese catalysts and propose additional ligand modifications that may improve catalytic performance. This characterization provides insight into the evolutionary benefit of nature employing manganese-based catalysts along with potential designs for efficient water oxidation catalysts as a natural hydrogen fuel source.